Expectations for a solar cell that directly converts sunlight energy into electric energy as the energy source for the next generation has rapidly grown these few years particularly from the standpoint of global environmental problems. Among the various types of solar cells employing a compound semiconductor or an organic material, a solar cell employing silicon crystal is now the main stream.
FIG. 26 represents a schematic sectional view of an example of a conventional solar cell. The solar cell has an n+ layer 11 formed at the light-receiving face of a p-type silicon substrate 10. A pn junction is formed by p-type silicon substrate 10 and n+ layer 11. An anti-reflection film 12 and a silver electrode 13 are formed on the light-receiving face of p-type silicon substrate 10. Further, a p+ layer 15 is formed at the back side of p-type silicon substrate 10, opposite to the light-receiving face. In addition, an aluminium electrode 14 and a silver electrode 16 are formed on the back side of p-type silicon substrate 10. The aforementioned pn junction corresponds to a photoelectric conversion portion at p-type silicon substrate 10.
FIG. 27 (a)-(i) represents an example of a fabrication method of a conventional solar cell. First, as shown in FIG. 27(a), a silicon ingot 17 obtained by dissolving p-type silicon crystal material in a crucible and recrystallizing the material is cut into silicon blocks 18. As shown in FIG. 27(b), a silicon block 18 is cut with a wire saw to produce a p-type silicon substrate 10.
Then, the surface of p-type silicon substrate 10 is etched using alkali or acid to remove a damage layer 19 generated during the slicing process of p-type silicon substrate 10 shown in FIG. 27(c). At this stage, microscopic asperities (not shown) can be formed at the surface of p-type silicon substrate 10 by adjusting the etching conditions. The asperities are advantageous in that reflection of sunlight incident on the surface of p-type silicon substrate 10 is reduced to allow the photovoltaic conversion efficiency of the solar cell to be improved.
Subsequently, as shown in FIG. 27(d), a dopant solution 20 containing a compound including phosphorus is applied on one main surface (hereinafter, referred to as “first main surface”) of p-type silicon substrate 10. By heating p-type silicon substrate 10 with dopant solution 20 applied for 5 to 30 minutes at the temperature of 800° C. to 950° C. to cause diffusion of phosphorus that is an n type dopant at the first main surface of p-type silicon substrate 10, an n+ layer 11 is formed at the first main surface of p-type silicon substrate 10, as shown in FIG. 27(e). The method of forming n+ layer 11 includes a method of vapor phase diffusion using P2O5 or POCl3 in addition to the method of applying a dopant solution.
Following the removal of a glass layer formed at the first main surface of p-type silicon substrate 10 at the time of phosphorus diffusion by an acid treatment, an anti-reflection film 12 is formed on the first main surface of p-type silicon substrate 10, as shown in FIG. 27(f). Known methods to form anti-reflection film 12 includes a method of forming a titanium oxide film by means of atmospheric pressure CVD, and forming a silicon nitride film by means of plasma CVD. In the case where phosphorus is to be diffused by the method of applying a dopant solution, the usage of a dopant solution containing the material of anti-reflection film 12 in addition to phosphorus allows simultaneous formation of n+ layer 11 and anti-reflection film 12. There are also cases where anti-reflection film 12 is formed after formation of a silver electrode.
As shown in FIG. 27(g), an aluminium electrode 14 is formed on the other main surface (hereinafter, referred to as “second main surface”) of p-type silicon substrate 10. In addition, a p+ layer 15 is formed at the second main surface of p-type silicon substrate 10. The formation of aluminium electrode 14 and a p+ layer 15 can be carried out as set forth below. For example, aluminium paste composed of aluminium powder, glass frit, resin and an organic solvent is applied by screen-printing and the like, followed by heat-treating p-type silicon substrate 10 for fusion of aluminium to generate an alloy with silicon, resulting in the formation of an aluminium-silicon alloy layer. Under this aluminium-silicon alloy layer, p+ layer 15 is formed. In addition, aluminium electrode 14 is formed on the second main surface of p-type silicon substrate 10. The difference in the dopant concentration between p-type silicon substrate 10 and p+ layer 15 causes a potential difference (acting as a potential barrier) at the interface between p-type silicon substrate 10 and p+ layer 15, which prevents optically-generated carriers from recoupling in the proximity of the second main surface layer of p-type silicon substrate 10. Accordingly, the short-circuit current (Isc) and the open circuit voltage (Voc) of the solar cell are both improved.
Then, a silver electrode 16 is formed on the second main surface of p-type silicon substrate 10, as shown in FIG. 27(h). Silver electrode 16 can be produced by printing silver paste composed of silver powder, glass frit, resin, and an organic solvent by means of screen-printing and the like, followed by heat-treating p-type silicon substrate 10.
Then, a silver electrode 13 is formed on the first main surface of p-type silicon substrate 10, as shown in FIG. 27(i). The pattern design such as the line width, pitch, thickness and the like of silver electrode 13 is crucial for the purpose of setting low the series resistance including the contact resistance with p-type silicon substrate 10 and also reducing the area where silver electrode 13 is formed to avoid reduction in the incident sunlight. An exemplified method of forming silver electrode 13 includes the steps of applying silver paste composed of silver powder, glass frit, resin, and an organic solvent on the surface of an anti-reflection film 12 by screen-printing, for example, and heat-treating p-type silicon substrate 10 to cause passage of the silver paste through anti-reflection film 12, allowing favorable electrical contact with the first main surface of p-type silicon substrate. This fire-through process is employed in a mass production line.
A solar cell structured as shown in FIG. 26 can be fabricated as set forth above. A solder coat may be applied on the surface of silver electrodes 13 and 16 by immersing p-type silicon substrate 10 having silver electrodes 13 and 16 formed in a molten solder bath. This solder coating step may be omitted depending upon the process. Furthermore, the solar cell fabricated as set forth above may be irradiated with pseudo sunlight using a solar simulator to measure the current-voltage (IV) characteristics of the solar cell to test the IV characteristics.
A plurality of solar cells are connected in series to constitute a solar cell string. Then, the solar cell string is sealed with a sealant to be offered for sale and usage in the form of a solar cell module.
FIG. 28(a)-(e) represents an example of a fabrication method of a conventional solar cell module. Referring to FIG. 28(a), an interconnector 31 that is a conductive member is connected on the silver electrode at the first main surface of solar cell 30.
Referring to FIG. 28(b), a row of solar cells 30 having interconnector 31 connected are arranged. Interconnector 31 connected to the silver electrode located at the first main surface of a solar cell 30 has its other end connected to the silver electrode located at the second main surface of another solar cell 30. Thus, a solar cell string 34 is produced.
Referring to FIG. 28(c), the solar cell strings are arranged and connected with each other by connecting in series an interconnector 31 protruding from both ends of a solar cell string with an interconnector 31 protruding from both ends of another solar cell string by means of a wiring material 33 that is a conductive member.
Referring to FIG. 28(d), the connected solar cell string 34 is sandwiched between EVA (ethylene vinyl acetate) films 36, identified as a sealing member, and then further sandwiched between a glass sheet 35 and a back film 37. The air bubbles present between EVA films 36 are removed by reducing the pressure, and heat treatment is applied. Accordingly, EVA film 36 is cured, whereby the solar cell string is sealed in the EVA. Thus, a solar cell module is produced.
Referring to FIG. 28(e), the solar cell module is placed in an aluminium frame 40. A terminal box 38 including a cable 39 is attached to the solar cell module. Then, the solar cell module produced as set forth above is irradiated with pseudo sunlight using a solar simulator to measure the current-voltage (IV) characteristics of the solar cell to test the IV characteristics.
The schematic plan view of FIG. 29 represents the configuration of silver electrode 13 formed on the first main surface of p-type silicon substrate 10, which will be the light-receiving face of the solar cell of FIG. 26. Silver electrode 13 is formed including one linear bus bar electrode 13a of a relatively large width, and a plurality of linear finger electrodes 13b of a relatively small width, extending from bus bar electrode 13a. 
The schematic plan view of FIG. 30 represents the configuration of aluminium electrode 14 and silver electrode 16 formed on the second main surface of p-type silicon substrate 10, identified as the back side of the solar cell of FIG. 26. Aluminium electrode 14 is formed nearly all over the second main surface of p-type silicon substrate 10, whereas silver electrode 16 is formed only at a portion of the second main surface of p-type silicon substrate 10. Silver electrode 16 that allows a solder coat to be applied may be required since it is difficult to apply a solder coat on aluminium electrode 14.
FIG. 31 schematically shows a sectional view of a solar cell string having the solar cells of the configuration of FIG. 26 connected in series. Interconnector 31 secured by means of soldering or the like to bus bar electrode 13a at the light-receiving face of a solar cell is fastened to silver electrode 16 located at the back side of another adjacent solar cell by means of soldering. In FIG. 31, the n+ layer, p+ layer and anti-reflection film are not depicted.
Patent Document 1: Japanese Patent Laying-Open No. 2005-142282